This invention relates to a heated galvanic-type solid electrolyte oxygen sensor, and more particularly to an improved and readily assemblable construction of such a sensor.
Solid electrolyte galvanic oxygen sensors essentially include an oxygen-ion-conductive ceramic body with porous electrodes on opposite faces of the body. One electrode is exposed to a reference source of oxygen. The other electrode is exposed to a source whose oxygen content is to be measured. A difference in oxygen partial pressure at the electrodes results in a corresponding electrode potential difference, providing a sensor output voltage.
The output voltage of such sensors can be used to measure oxygen or unburned combustibles in combustion gases produced by an internal combustion engine. This voltage can be used in monitoring and controlling the combustion process, as disclosed in U.S. Pat. Nos. 3,616,274 Eddy, 3,844,920 Burgett et al and U.S. Ser. No. 787,900 Howarth, filed Apr. 15, 1977, now U.S. Pat. No. 4,129,099.
The solid electrolyte of such a sensor must be heated to an elevated temperature to obtain an appreciable output voltage. Also, sensor output voltage varies directly with changes in temperature, especially at lower operating temperatures. Combustion gases can be used to heat the sensor to operating temperatures but such gases vary widely in temperature, particularly when from an internal combustion engine. The aforementioned U.S. Pat. No. 3,616,724 Eddy discloses sensor temperature compensating means that includes a surrounding resistance heater. U.S. Pat. No. 3,815,560 Wahl et al discloses a surrounding resistive heater to maintain an electrolyte tube at high temperatures where its output voltage is least affected by temperature change. The aforementioned U.S. Ser. No. 787,900 Howarth discloses doping the solid electrolyte with iron oxide for temperature compensation. It additionally discloses disposing a resistance heater inside a solid electrolyte tube for maintaining the sensor at higher operating temperatures and for supplemental heating on start-up.
I have found a better way to include the heater in the sensor. For automotive applications particularly, the heated sensor should be rugged and reliable. In addition, the construction should be readily manufacturable for lower cost. Exposing the heater to the exhaust gas stream, around a solid electrolyte tube, can adversely affect both reliability of the heater and heating efficiency. Nesting the heater inside a closed end of a solid electrolyte tube, where it is isolated from the exhaust gas, has been previously proposed. However, such constructions use the inner surface of the tube for alignment and support, and involve complex mechanical and/or electrical arrangements. I believe that such constructions are unduly costly and raise questions of reliability and ruggedness.
I have found a new way to incorporate a heater in the sensor, that is particularly useful for automobile exhaust gas sensing. It involves initially forming a subassembly of a heater and a reference electrode terminal in which the heater is prealigned with respect to the solid electrolyte and isolated from the exhaust gas. The subassembly is simple, rugged, and reliable, yet readily mass produced. An elongated heater can readily be incorporated in a subassembly with the center terminal of an oxygen sensor such as disclosed in U.S. Pat. No. 3,844,920 Burgett et al. Such a heated sensor is rugged, efficiently heated, and yet readily manufacturable in high volume for lowest cost.